Porous materials such as silicas and zeolites have been examined as adsorbents in water-based adsorption heat pumps and heat storage devices. Silicas generally display poor water exchange capacities, in part due to low hydrophilicity. Water exchange capacity, the amount of water adsorbed and desorbed per gram adsorbent during the working and regeneration cycles, is an important feature since it relates to the amount of heat transferred in a given cycle. More hydrophilic zeolites and zeolite-type materials have been shown to exhibit increased water exchange capacities, as compared to silicas but the increase in hydrophilicity leads to higher desorption temperatures which may surpass those provided by low-temperature waste heat or solar collectors.
While much effort has been placed toward tuning the structure and function of metal organic frameworks (MOFs) for adsorption and separation of gases such as H2, CH4, and CO2, less attention has been given toward modifying their water adsorption properties. This is due, at least in part, to the hydrolytic instability of many MOFs. The water adsorption behaviors of the few water-stable MOFs containing carboxylate-based ligands have been studied in some detail and evidenced characteristics desirable in heat pump adsorbents. While some metal-organic frameworks in general have attracted interest as adsorbents in water-based adsorption heat pumps, many have low water loading capacities.
Accordingly, improved compositions and methods are needed.